Many industrially important processes require water removal apparatus and processes which consume low amounts of energy. More specifically, dewatering algae is a major hurdle in the economic production of algae derived biofuels, pharmaceuticals, and nutraceuticals.
Oil-producing microalgae are an attractive alternative to conventional fuel sources as they grow extremely rapidly, have the potential to produce as much as 100 times more oil per hectare of land area than land-based crops, and, if properly exploited, can serve as a biofuel source that is economical, sustainable, reduces global warming, reduces the need to displace conventional food crops, and provides energy independence.
Oleaginous microalgae use photosynthesis to capture carbon from the air (in the form of carbon dioxide) to produce the various cellular chemicals needed to live. One class of cellular chemicals of particular interest is lipids. When properly grown, some of the oleaginous microalgae species can reach up to eighty percent lipid content (by cell dry weight), which includes hydrocarbons (e.g., β-carotene, terpenoids), triglycerides, and other minor components (e.g., sterols, glycolipids, and phospholipids). While some of these hydrocarbons can often be used directly a diesel-grade motor fuel, many of these lipids, through post-processing, can also be transformed into other types of organic compounds and feedstocks.
There are two main approaches to growing algae in bulk: closed bioreactors and open ponds. Bioreactors are often 10-100 times more costly to build and operate as compared to open ponds, which limits their usefulness outside of products that require tight controls and are highly value-added (e.g., pharmaceuticals and nutraceuticals). However, although cheaper, open ponds have some potential problems with contamination and overall biomass concentration. One approach that combines some of the advantages of the bioreactor and open pond layouts consists of using large ponds covered with a canopy. This hybrid system protects the algae from many of the harmful effects of the environment while still allowing for large, pond-like growth areas. However, a major challenge for production of algae derived biofuels still remains: the harvesting of the lipids from the algae in a large scale, cost-effective manner.
In order to use algae-derived hydrocarbons as a fuel or as a feedstock for fuel production, pharmaceutical production, and/or nutraceutical production, the algae must be separated from an aqueous growth medium (an aqueous solution containing trace elements such as nitrogen, phosphorous, etc.) using apparatuses and processes which require substantially less energy input than the energy content of the algae biomass or, alternately, less energy investment than the value of the product. For biofuel production, the target energy requirement, as stated by the U.S. Department of Energy, is that the energy must be less than or equal to 10% of the energy content of the biomass.
Several suspended particle separation technologies are known in the art and include centrifugation, flotation, filtration, sedimentation, and the like. However, these either consume too much energy, operate at low throughput, or require the addition of chemicals that require subsequent removal. Current methods to extract the oil from algae are solvent extraction (for example, supercritical CO2, hexane, benzene) and an expeller press. Solvent extraction produces hazardous waste as a byproduct, and both solvent extraction and expeller press processes tend to be energy intensive, reducing the net energy yield of the oil. Solvent inputs and press machinery are also quite costly.
Filtration is a low energy consumption separation technology, and filtration is often combined with other separation technologies in a hybrid concept. A significant limitation of filtration is that as the separation process proceeds, the filter medium becomes clogged with the suspended particles. At this point the separation of suspended particles from the solution becomes slow, and the filter medium must be replaced, backwashed, or otherwise rejuvenated to re-establish acceptable solution flow rates and/or differences in head. As the filter medium becomes clogged with suspended particles, the operating and capital cost of filtration-based separation technology becomes higher. Consequently, there is a need for filtration technology in which clogging of the filter medium with suspended particles is minimized or eliminated.